This invention relates to superconducting machines.
Superconducting air-core, synchronous electric machines have been under development since the early 1960""s. The use of superconducting windings in these machines has resulted in a significant increase in the field electromotive forces generated by the windings and increased flux and power densities of the machines.
Early superconducting machines included field windings wound with low temperature superconductor (LTS) materials, such as NbZr or NbTi and later with Nb3Sn. The field windings were cooled with liquid helium from a stationary liquifier. The liquid helium was transferred into the rotor of the machine and then vaporized to use both the latent and sensible heat of the fluid to cool the windings. This approach proved to be viable for only very large synchronous machines. With the advent of high temperature superconductor (HTS) materials in the 1980""s, the cooling requirements of these machines were greatly reduced and smaller superconducting machines were realizable.
In superconducting machinery, efficiency and size are of critical importance. One way of reducing the size of a superconducting machine is to minimize the air gap between the field windings and the stator windings. Unfortunately, since superconducting rotor windings typically utilize some form of metallic shielding to minimize the detrimental affect of asynchronous fields in the stator windings, as this air gap is reduced, the stator windings get closer to this metallic shielding and subtransient reactance is reduced. This reduction in subtransient reactance results in higher levels of braking torque and stator current being experienced by the superconducting machine during fault conditions.
According to an aspect of this invention, a superconducting rotating machine includes a stator assembly. This stator assembly includes at least one stator coil assembly having a first predefined length. A rotor assembly is configured to rotate within this stator assembly and is spaced from the stator assembly by a gap. The rotor assembly includes at least one superconducting rotor winding assembly that, in operation, generates a magnetic flux linking the stator assembly. The rotor assembly includes an asynchronous field filtering shield having a second predefined length that is less than the first predefined length. This shield is positioned between the stator assembly and the rotor assembly.
One or more of the following features may also be included. The asynchronous field filtering shield is constructed of a non-magnetic material, such as copper or aluminum. The first predefined length is a differential length greater than the second predefined length, such that this differential length may be a percentage of the first predefined length, a percentage of the second predefined length, or a fixed length. The stator coil assembly is constructed using copper non-superconducting material. The rotor winding assembly is constructed using a high-temperature superconducting material, such as: thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide; mercury-barium-calcium-copper-oxide; and yttrium-barium-copper-oxide. The superconducting machine further includes a refrigeration system for cooling the superconducting rotor winding assembly. The stator coil assembly includes a center section and a pair end-turn sections positioned at distal ends of the center section. The asynchronous field filtering shield is positioned between the center section of the stator coil assembly and superconducting rotor winding assembly. The end-turn sections of the stator coil assembly extend beyond the asynchronous field filtering shield.
According to a further aspect of this invention, a method of maintaining a sufficient level of subtransient reactance in a superconducting machine includes producing a stator assembly that includes at least one stator coil assembly having a first predefined length. The method produces a rotor assembly configured to rotate within the stator assembly and spaced from the stator assembly by a gap. This rotor assembly includes at least one superconducting rotor winding assembly that, in operation, generates a magnetic flux linking the stator assembly. The method positions an asynchronous field filtering shield, having a second predefined length that is less than the first predefined length, between the stator assembly and the rotor assembly. Further, if the size of the superconducting machine is to be minimized, the method may also reduce the gap between the stator assembly and the rotor assembly to the minimum allowed by mechanical considerations.
One or more of the following features may also be included. The method includes rigidly affixing the asynchronous field filtering shield to the rotor assembly or the stator assembly. The stator coil assembly includes a center section and a pair end-turn sections positioned at distal ends of the center section. The positioning an asynchronous field filtering shield includes: positioning the asynchronous field filtering shield between the center section of the stator coil assembly and the superconducting rotor winding assembly; and extending the end-turn sections of the at least one stator coil assembly beyond the asynchronous field filtering shield.
According to a further aspect of this invention, a stator assembly is configured to accept a superconducting rotor assembly having an asynchronous field filtering shield of a first predefined length. The stator assembly includes at least one stator coil assembly having a second predefined length, which is greater than the first predefined length. The shield is positioned between the stator assembly and the rotor assembly.
According to a further aspect of this invention, a superconducting rotating machine includes a stator assembly. This stator assembly includes at least one stator coil assembly having a center section and a pair end-turn sections positioned at distal ends of the center section. A superconducting rotor assembly is configured to rotate within the stator assembly and is spaced from the stator assembly by a gap. The rotor assembly includes an asynchronous field filtering shield positioned between the stator assembly and the rotor assembly. At least one of the end-turn sections of the at least one stator coil assembly is flared radially away from the asynchronous field filtering shield. This creates an expanded gap between the end-turn sections and the asynchronous field filtering shield.
One or more of the following features may also be included. The expanded gap is two to three times larger than the gap. The at least one stator coil assembly includes an inner surface and an outer surface. The inner surface is positioned proximate the asynchronous field filtering shield. The superconducting machine further includes a flux return path positioned circumferentially about the outer surface of the end turn sections of the at least one stator coil assembly. The flux return path is constructed of a magnetic material. The magnetic material is laminated sheet steel. The asynchronous field filtering shield is constructed of a non-magnetic material. The non-magnetic material is copper. The non-magnetic material is aluminum. The at least one stator coil assembly is constructed using a copper non-superconducting material. The superconducting rotor assembly includes at least one superconducting rotor winding assembly which, in operation, generates a magnetic flux linking the stator assembly. The at least one superconducting rotor winding assembly is constructed using a high-temperature superconducting material. The high temperature superconducting material is chosen from the group consisting of: thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide; mercury-barium-calcium-copper-oxide; and yttrium-barium-copper-oxide. The superconducting machine further includes a refrigeration system for cooling the superconducting rotor assembly. Both of the end-turn sections of the at least one stator coil assembly are flared radially away from the asynchronous field filtering shield. One of the end-turn sections of the at least one stator coil assembly is flared radially away from the asynchronous field filtering shield and the other end-turn section is non-flared. The non-flared end-turn section is coterminous with the asynchronous field filtering shield. The non-flared end-turn section extends past the asynchronous field filtering shield.
According to a further aspect of this invention, a superconducting rotating machine includes a stator assembly. The stator assembly includes at least one stator coil assembly having a first predefined length. The at least one stator coil assembly includes a center section and a pair end-turn sections positioned at distal ends of the center section. A superconducting rotor assembly is configured to rotate within the stator assembly and is spaced from the stator assembly by a gap. The rotor assembly includes an asynchronous field filtering shield having a second predefined length which is less than the first predefined length. The shield is positioned between the stator assembly and the rotor assembly. At least one of the end-turn sections of the at least one stator coil assembly is flared radially away from the asynchronous field filtering shield. This creates an expanded gap between the end-turn sections and the asynchronous field filtering shield.
One or more of the following features may also be included. The expanded gap is two to three times larger than the gap. The at least one stator coil assembly includes an inner surface and an outer surface. The inner surface is positioned proximate the asynchronous field filtering shield. The superconducting machine further includes a flux return path positioned circumferentially about the outer surface of the end turn sections of the at least one stator coil assembly. The flux return path is constructed of a magnetic material. The magnetic material is laminated sheet steel. The asynchronous field filtering shield is constructed of a non-magnetic material. The non-magnetic material is copper. The non-magnetic material is aluminum. The first predefined length is a differential length greater than the second predefined length. The differential length is a percentage of the first predefined length, a percentage of the second predefined length, or a fixed length. The at least one stator coil assembly is constructed using a copper non-superconducting material. The superconducting rotor assembly includes at least one superconducting rotor winding assembly which, in operation, generates a magnetic flux linking the stator assembly. The at least one superconducting rotor winding assembly is constructed using a high-temperature superconducting material. The high temperature superconducting material is chosen from the group consisting of: thallium-barium-calcium-copper-oxide; bismuth-strontium-calcium-copper-oxide; mercury-barium-calcium-copper-oxide; and yttrium-barium-copper-oxide. The superconducting machine further includes a refrigeration system for cooling the at least one superconducting rotor winding assembly. Both of the end-turn sections of the at least one stator coil assembly are flared radially away from the asynchronous field filtering shield. One of the end-turn sections of the at least one stator coil assembly is flared radially away from the asynchronous field filtering shield and the other end-turn section is non-flared. The non-flared end-turn section is coterminous with the asynchronous field filtering shield. The non-flared end-turn section extends past the asynchronous field filtering shield.
According to a further aspect of this invention, a method of maintaining a sufficient level of subtransient reactance while decreasing the size and cost of a superconducting machine includes producing a stator assembly. The stator assembly includes at least one stator coil assembly having a center section and a pair end-turn sections positioned at distal ends of the center section. The method produces a superconducting rotor assembly that is configured to rotate within the stator assembly and spaced from the stator assembly by a gap. The method then positions an asynchronous field filtering shield between the stator assembly and the rotor assembly. The method flares the end-turn sections of the at least one stator coil assembly radially away from the asynchronous field filtering shield, thus creating an expanded gap between the end-turn sections and the asynchronous field filtering shield. The method then reduces the gap between the stator assembly and the rotor assembly to the minimum allowed by mechanical considerations.
One or more of the following features may be included. The method rigidly affixes the asynchronous field filtering shield to the rotor assembly.
One or more advantages can be provided from the above aspects of the invention. The efficiency of superconducting machines can be increased by reducing the air gap between the stator assembly and the rotor assembly. This efficiency can be increased while maintaining acceptable levels of subtransient reactance. By maintaining an acceptable level of subtransient reactance, the braking torque experienced during system faults by this efficient superconducting machine can be maintained at a reasonable level. This reduction in braking torque simplifies the design criteria associated with the rotor""s torque tube.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.